An anticorrosive paint mainly containing a zinc powder and chromic acid had been much used as a paint to control corrosion of a member having a metallic surface such as a steel member. This paint has superior storage stability because the paint can keep the zinc powder stable for a long period and because of a passivation effect that hexavalent chromium provides. In addition, a coating formed from a paint having a zinc powder prevents corrosion of a metal substrate such as steel because of the known effect of zinc of providing sacrificial protection. Therefore, the coating exhibits excellent rust-proof properties.
However, there has recently been concern of environmental pollution and health damage to the human body caused by the toxicity of hexavalent chromium, and a movement to impose legal restrictions on hazardous metals such as hexavalent chromium has developed. In accordance with this movement, many industries contemplate not using hazardous metals such as hexavalent chromium at all. Therefore, in the field of rust-proof paints, there is a strong desire to prepare a paint totally free from hazardous metals such as chromium.
Such rust-proof paints which do not contain chromium include paints having a zinc powder and a binder component dispersed or dissolved in a solvent, i.e. zinc-rich paints. The binder component of zinc-rich paints can be categorized as an organic type or an inorganic type. Because an inorganic binder component is superior from the viewpoint of durability, a zinc-rich paint having an inorganic binder component is used for an undercoating agent in heavy-duty coatings, such as for a ship or a bridge.
However, a coating formed from such an inorganic zinc-rich paint has a tendency to have voids therein, and it is difficult to control the thickness of the coating. The following means have been disclosed to overcome such problems.
Patent Document 1 discloses a zinc-rich paint which additionally contains calcium carbonate in the form of whiskers with a major axis of 20 to 30 micrometers. The added whiskers act to prevent from forming cracks in a coating of the paint
Patent Document 2 discloses a zinc-rich paint containing an alkyl silicate resin for which the ratio of the weight-average molecular weight to the number average molecular weight is 40 or less, with the morpholine gel time of the paint being at most 60 seconds. It is explained in that document that the phenomenon of elongation of cracks and connection of cracks to voids is inhibited because the curing period of the paint is short.    Patent Document 1: JP1999-293200A    Patent Document 2: JP2004-359800A
Although the means described in the above patent documents are indeed effective for a thick coating formed from a zinc-rich paint, they cannot provide a paint that stably forms a thin coating having a thickness of about 10 micrometers and excellent anticorrosive properties.
Such a thin coating having excellent anticorrosive properties is mainly applied to office equipment, electric appliances, vehicles, and the like. Specifically, it is applied to fasteners such as bolts and nuts; attachments such as clamps and clips; and press molded parts such as plates, housings, hinges, and panels. These members are subject to strong shear stresses during their manufacture and assembly, even though they are manufactured to tight tolerances. Therefore, their coatings must have a high film strength and a high level of adhesive properties.
An effective measure for meeting the above requirement for coatings is a baking treatment at a high temperature. However, when a zinc-rich paint is subjected to baking treatment at about 300 degrees C., an organic silicon compound which functions as a binder component in the paint rapidly shrinks. Therefore, even when the means described in the above-mentioned patent documents are applied, extension of cracks in a coating formed from a zinc-rich paint cannot be stopped, and cracks occasionally extend into the substrate.
Accordingly, it is an important technological goal to provide a rust-proof paint which does not contain a hazardous metal such as chromium and is able to form a thin coating in which crack generation is inhibited even after a baking treatment at a high temperature.